Superheterodyne receiver



Viv/f O Nov. 24, 1936.

C. W. HANSELL SUPERHETERODYNE RECEIVER Filed Jan. 27, 1934 3 Sheets$hovet 1 Hf /MPU AUD/0 0077007' .4 CAL INVENTOR C. W. HA

ATTORNEY Nov. 24, 1936. c. w. HANsELL SUPERHETERODYNE RECEIVER Filed Jan. 27, 1934 3 Sheets-Sheet 2 khlkbb In R E N E m n l v A m 0cm Lm UVW NOV. 24, 1936. Q w HANSELL 2,062,004

SUPERHETERODYNE RECEIVER Filed Jan. 27, 1934 3 Sheets-Sheet 3 '"1 i .f-ll* I I IN INVENTOR c.w. HAN LL AT TO RN EY Patented Nov. 24, 1936 UNITED STATES PATENT OFFICE SUPERHETERODYNE RECEIVER Clarence W. Hansell, Rocky Point, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application January 27, 1934, Serial No. 708,540

6 Claims.

This invention relates to the reception of radio frequency signals and has for its chief purpose the provision of an improved type of detector for beating an incoming high frequency signal down to an intermediate frequency signal in a superheterodyne type of receiver. Briefly stated, the invention consists of the application of grid detection, of the type commonly used for beating down to audio frequencies, in detectors used for beating down to intermediate frequencies. It consists essentially in using a parallel tuned circuit tuned to the intermediate frequency to serve the same function as the grid leak in the common types of detector. Various other features such as regeneration and the addition of a grid leak as well as a tuned circuit, together with the combination of the new type of detector with various receiving circuits, are part of the invention.

In the usual superheterodyne receiver the first detector is almost always of the bias type, which is commonly known as a power detector. This type lof detector gives fairly satisfactory results when a relatively large amount of energy is supplied by the local oscillator for the purpose of making the detector tube more sensitive, but it is subject to limitations in that for high frequency reception a relatively high intermediate frequency must be used and all of the intermediate frequency selectivity is obtained in the circuits following the first detector. In order that this selectivity may be sufliciently great it is necessary to use a relatively large number of intermediate frequency stages. If intermediate frequency selectivity could be applied ahead of the detector tube as is done in this invention, then it would be possible to eliminate one stage of intermediate frequency amplifier without reducing the overall selectivity of the intermediate frequency system. This would result in economies in receiver costs in many cases.

In existing types of superheterodyne receivers, I know of no instances where regeneration is used in intermediate frequency stages for the purpose of increasing the receiver sensitivity and selectivity. Accordingly, one of the objects of this invention is to add this regeneration for the purpose of obtaining either much improved sensitivity and selectivity or of making it possible to eliminate intermediate frequency amplifier stages in a manner to reduce the cost of construction and operation. of the receivers. The need for these improvements is particularly great in high frequency reception because the value of intermediate frequencies must be quite high. It is quite well known that the intermediate frequency should have a value at least as great as half the width of pass band of the circuits in the high frequency stages ahead of the first detector. Since the selectivity of the intermediate frequency circuits is decreased as the Value of this frequency is increased, due to designing and operating receivers for higher and higher transmitted frequencies, it becomes desirable to improve the selectivity of each individual circuit in the intermediate frequency system. This is done by the system of regeneration to be disclosed here.

The novel features of my invntion have been pointed out with particularity in the claims appended hereto. The nature of my invention and circuit arrangements for carrying out the same will be understood from the following detailed description and therefrom when read in connection with the attached drawings in which Figure 1 illustrates a circuit for beating together oscillations of different frequency to produce a beat note of audible frequency.

Figures 2 to 4 inclusive illustrate circuits arranged in accordance with the present', invention for mixing two or more carrier frequency Waves to produce a beat note which is above audible frequency. The carrier frequencies may be of ultra high frequency and the beat note produced may be a high intermediate frequency.

Figures 5 and 6 illustrate superheterodyne receivers including the novel heterodyne arrangement of the present invention and other novel features.

In Figure 1 is shown a common form of regenerative detector for producing beat frequency outputs at audio frequencies. In this detector circuit, l is tuned to the incoming radio frequency signal and acts as a transformer circuit for applying radio frequency potentials to the grid of the vacuum tube. The energy in circuit I is increased by means of regeneration introduced through coupling coil 2. The amount of current at the operating frequency which passes through coupling coil 2 is varied by means of adjustable condenser 3 and this serves to govern the amount of regeneration. The radio frequency potentials applied to the grid of the tube cause rectified current to flow from grid to filament and this produces a drop in the grid leak resistor 4, the amount of which varies in accordance with the modulation of the incoming signal and the local oscillator. Condenser 5 in shunt with the grid leak is used to short circuit radio frequency current but is substantially an open circuit for modulating frequencies. The varying amount of rectication in the grid, by producing the variations in grid bias potential through the drop in resistance 4, causes a variation in the plate current of the tube to give an audio output through the output transformer, which is used to operate headphones, loudspeakers, tape recorders, etc. through suitable amplifiers.

When it is attempted to use the circuit of Figure l to produce outputs from the detector tubes at radio frequencies such as are produced by the first detector of a superheterodyne receiver, difiiculty is encountered because it is hard to make the capacity across grid leak 4 sufficiently small so that the potential drop through resistor 4 follows the intermediate frequency variations in grid rectification. Even though with capacity of condenser reduced to zero, which would be undesirable because of radio frequency losses in the grid leak, the distributed capacity of circuit l and of the vacuum tube would still be sufficient to require the use of a relatively low value of resistance in the grid leak with consequent loss in sensitivity.

To overcome this dimculty in accordance with the present invention, I use a circuit such as that shown in Figure 2. In this circuit, I is the input circuit tuned to the incoming signal modulated wave. 2 is the coupling inductance to produce high frequency regeneration. Condenser 3 permits control of the high frequency regeneration. 4 is a grid leak resistance and 5 is a grid condenser having a. relatively very large value of capacity while 6 is a circuit tuned to the intermediate frequency. The output circuit 1 is also tuned to the intermediate frequency and is coupled to the utilization circuit. With a circuit such as that shown in Figure 2, the local oscillator is set at a frequency different from the incoming signal by an amount equal to the intermediate frequency desired in the output of the detector. Beats between the local oscillator and the signal modulated carrier then produce variations at an intermediate frequency rate in the rectification action of the grid circuit of the vacuum tube and consequently variations in the grid direct current. The latter variations build up an oscillating potential in circuit 6, which causes the plate current of the vacuum tube to vary at the intermediate frequency. By tuning circuit 1 to the beat frequency, a high output impedance is built up across the plate circuit of the tube and maximum intermediate frequency energy output is obtained. Since the potentials set up in circuit 6 are oscillating in character and reverse in polarity at half cycle intervals, it is desirable that the grid leak 4 and condenser 5 be used so that the grid has a steady biasing potential of a value equal to or greater than the peak value of voltage fluctuation in circuit 6. However, since condenser 5 is made of large capacity, there are substantially no fluctuations in potential across resistance 4 and the function of the grid leak and condenser is only to maintain a steady direct current bias.

As an alternative to the detector shown in Figure 2 I may use that shown in Figure 3, in which regeneration is accomplished at the intermediate frequency instead of at the high frequency. In this case there is coupling between the intermediate frequency tuned circuit of the plate and the intermediate frequency circuit of the grid. The amount of this coupling may be held fixed in actual practice since a constant intermediate frequency may be used and the tuning of the receiver is accomplished by simply varying the frequency of the local oscillator and of radio frequency ampliiiers between the antenna circuit and the detector, if these amplifiers are used. It will be noted that the use of regeneration at the intermediate frequency does not require the addition of any control to the receiver but it has the decided advantage that the detector sensitivity and frequency selectivity is greatly increased. As a consequence, improved operation is obtained through reducing the number of intermediate frequency amplifiers and through limiting the pass band of the receiver, which might admit noise.

In Figure 4 I have shown still another circuit in which regeneration is used at both high and intermediate frequencies. In this case I would expect to iix the amount of regeneration at the intermediate frequency but to allow the operator control of the amount of regeneration at the high frequency. The addition of the intermediate frequency regeneration need not be taken into account by the operator and it is possible that he need not even be aware of its use.

Figure 5 is a schematic diagram illustrating one application of the intermediate frequency tuned circuit in the grid of a detector as applied to a complete superheterodyne receiver. The features of the circuits of this heterodyne receiver are well known in the prior art except as otherwise indicated hereinafter. The first detector and the oscillator is as shown in the drawings arranged in accordance with the present invention. A loop circuit 6 including a condenser and an inductance in parallel is connected in series with the grid leak and condenser between the cathode and control grid of the first detector as shown. The beat frequency oscillator is coupled to the first detector circuits as in the modiiications described hereinbefore. Fixed regeneration at the intermediate frequency and controllable regeneration at high frequency may be obtained as indicated by coupling the plate circuit of the first detector to the input circuit l and to the tuned circuit 6. The degree of regeneration at high frequency thus provided may be regulated by the variable condenser C connected between one side of this circuit and ground as shown. The second detector may include a radio frequency inductance RFL and variable condenser VC connected between the anode and control grid as shown. This variable condenser may be adjusted to produce regeneration if desired or the circuits may be modified and may be adjusted to neutralize the capacity between the anode and grid electrode of the detector in accordance with methods already well known in the art.

The filament heating leads are filtered as are the grid and anode energizing circuits. An electrostatic shield may be interposed between the antenna circuit and the radio frequency amplifier RF, which may be shielded from the remaining portions of the circuit as shown.

Although the invention is particularly adapted to ultra high frequency signals, that is, signals of frequency of which are above the broadcast range, it will be understood that the invention is fully applicable to heterodyne receivers to be used for receiving broadcast signals. Furthermore it will be understood that any types of tubes either directly or indirectly heated may be utilized and that the tubes may be either of the vtriode type or multiple grid electrode type. For example, my invention may be utilized in a commercial broadcast receiver as illustrated in Figure 6. In this receiver the grid circuit of the rst detector includes, as shown, a. loop circuit 6 comprising an inductance and capacity in parallel, in series with the usual grid leak and condensers 4 and 5. 'Ihis circuit may be tuned by the capacity to the intermediate frequency produced in the first detector and when so tuned the potential drop through the grid leak and condenser and tuned loop combination, due to grid rectification, follows faithfully the intermediate frequency variations which in turn cause the plate current to vary at an intermediate frequency. This, of course, increases the selectivity of the receiver so that the number of intermediate frequency amplifier stages may be reduced. In this arrangement controllable regeneration at the intermediate frequency is obtained by connecting a variable condenser between one terminal of the inductance in the anode of the first detector and the input circuit as shown.

Although I have shown inductive coupling for obtaining the regeneration in the circuits shown in the figures, it will be evident to anyone skilled in the art that I might also have used capacity coupling or conductive coupling through blocking condensers.

It will also be evident that the intermediate frequency grid circuit made up of a coil and condenser might be replaced by a coil of such dimensions that it in combination with the other circuits would be resonant without the necessity for the variable condenser which I have shown. In this case it is assumed that in the adjustments of the receiver, to make the intermediate frequency plate circuit tune for the same frequency as the grid circuit, it would only be necessary to adjust the condenser in the plate circuit.

Having thus described my invention and the operation thereof, what I claim is:

1. Means for producing oscillations of frequency which is equal to the difference between two signal oscillations comprising a thermionic tube having a control grid and anode and cathode, a circuit tuned to the difference frequency connected between said anode and cathode, a circuit for impressing said two signal oscillations on the control grid of said tube, means for varying the potential of said control grid at the difference frequency rate comprising a circuit tuned to the difference frequency connected between the control grid and cathode, and a feed back circuit coupling said rst named tuned circuit to said last named circuit.

2. Means for producing oscillations of frequency which is equal to the difference between two signal oscillations comprising a thermionic tube having a control grid and anode and cathode, a. circuit tuned to the difference frequency coupled between said anode and cathode, a circuit for impressing said two signal frequencies on the control grid of said tube, means for vary- QUCA Uil ing the potential of said control grid at the intermediate frequency rate comprising a circuit tuned to the intermediate frequency connected between the control grid and cathode, and coupling between the first named circuit and the last named circuit for producing regeneration in the difference frequency.

. 3. A frequency converting circuit for superheterodyne receivers and the like comprising a thermionic tube provided with an input circuit and an output circuit, a source of signal energy coupled to said input circuit, a source of heterodyning oscillations, means for combining within the tube oscillations from the last named source and the signal energy to thereby produce intermediate frequency energy, a resonant circuit tuned to the intermediate frequency and a grid leak and condenser arrangement in series in said input circuit, a resonant circuit in said output circuit said last named resonant circuit being tuned to the intermediate frequency and coupling between the output circuit and the input circuit for producing regeneration.

4. A frequency converting circuit for superheterodyne receivers and the like comprising a thermionic tube provided with an input circuit and an output circuit, a source of signal energy coupled to the input circuit, a source of heterodyning oscillations, means for combining, within the tube, oscillations from the last named source and the signal source to thereby produce intermediate frequency energy, a parallel tuned circuit connected in said output circuit, a parallel tuned circuit connected in said input circuit, both said two last named circuits being tuned to the intermediate frequency and means for coupling said two parallel tuned circuits.

5. A frequency converting circuit for superheterodyne receivers and the like comprising a thermionic tube provided with an input circuit and an output circuit, a source of signal energy coupled to the input circuit, a source of heterodyning oscillations, means for combining, within the tube, oscillations from the last named source and the signal energy to thereby produce intermediate frequency energy, said input circuit including a pair of anti-resonant circuits connected in series, one of said circuits being tuned to the frequency of the signal energy and the other thereof being tuned to the intermediate frequency, said output circuit including a parallel tuned circuit tuned to the intermediate frequency, and coupling between the last named circuit and one of said anti-resonant circuits.

6. A system as described in the next preceding claim further characterized by that means are provided for coupling the parallel tuned circuit to both of the anti-resonant circuits.

CLARENCE W. HANSELL. 

